Fossil fuels such as petroleum raise concerns about future depletion and have a problem of carbon dioxide emission causing global warming. In recent years, spread of photovoltaic power systems has been expanded, and they are expected as an energy source alternative to fossil fuels such as petroleum because of increased environmental awareness and low-priced systems in particular.
General solar cells are categorized into bulk solar cells and thin-film solar cells. The bulk solar cells are those produced with the use of single-crystal and polycrystalline silicons or bulk crystalline semiconductors including solar cells of gallium arsenide compounds, and mass production technology has been already established for most of such solar cells. Recently, however, the bulk solar cells have had problems of lack of raw materials due to rapid increase of production volume and difficult cost reduction.
As for the thin-film solar cells, in contrast, quantity of semiconductors to be used can be reduced considerably. Therefore, they attract attention as a next generation of solar cells having a possibility of resolution of the problem of lack of raw materials and significant cost reduction. Specifically, a thickness of a semiconductor layer of the thin-film solar cells is not more than 10 μm to several μm, whereas a thickness of the bulk solar cells is several hundreds μm. Generally, structures of the thin-film solar cells can be categorized into the following two types.
That is, they are categorized into either a superstrate type in which a transparent conductive layer, a photoelectric conversion layer, and a back electrode layer are laminated in this order on a transparent substrate, and light enters from a side of the transparent substrate; or a substrate type in which a back electrode layer, a photoelectric conversion layer, a transparent conductive layer, and a metal grid electrode are laminated in this order on a non-transparent substrate, and light enters from a side of the metal grid electrode.
Since the amount of semiconductors to be used is small in the thin-film solar cells as described above, technologies for effectively utilizing light that enters the semiconductor layer is very important in order to obtain high conversion efficiency. Technologies for that purpose include an optical confinement technology. The optical confinement technology is a technology to enhance photoelectric conversion efficiency by increasing the amount of light absorption by extending a substantial optical path length in the photoelectric conversion layer by forming a structure in which light is refracted and scattered at an interface between the photoelectric conversion layer and a material having a different index of refraction.
In addition, adoption of a structure of a stacked photoelectric conversion device is also a technology for effective use of incident light. The structure of the stacked photoelectric conversion device is a structure for splitting an incident light spectrum and receiving the split light spectrum in a plurality of photoelectric conversion layers, and by stacking a plurality of photoelectric conversion layers that use a semiconductor material having a bandgap suitable for absorbing the respective wavelength bands in decreasing order of bandgap from a light entrance side, it is possible to absorb light having a shorter wavelength in the photoelectric conversion layer having a larger bandgap and light having a longer wavelength in the photoelectric conversion, layer having a smaller bandgap, respectively.
Therefore, sunlight having a wider wavelength band can contribute to the photoelectric conversion compared with a device provided with one photoelectric conversion layer, and therefore it becomes possible to enhance the photoelectric conversion efficiency. Here, since a plurality of photoelectric conversion layers are connected in series in the stacked photoelectric conversion device, the open end voltage will be used without waste as the total of voltages generated in each photoelectric conversion layer, but the short circuit current density will be limited by the smallest value of photocurrents generated in each photoelectric conversion, layer. Therefore, it is an important factor for unwasted utilization of energy of the incident light to equalize photocurrent values generated in each photoelectric conversion layer.
While it is common to control the film thickness of each photoelectric conversion layer as a method for equalizing the photocurrent values generated in each photoelectric conversion layer, it is also known to control the amount of light that enters each photoelectric conversion layer by providing an interlayer between two adjacent photoelectric conversion layers. When the interlayer is provided, some of light that has reached the interlayer is reflected, and the rest of the light is allowed to pass through. Therefore, there is an effect of controlling the amount of incidence light to each photoelectric conversion layer, that is, the amount of the incidence light to a photoelectric conversion layer located at the light entrance side with respect to the interlayer (top cell) is increased, while the amount of the incidence light to a photoelectric conversion layer located at a side opposite to the light entrance side (bottom cell) is decreased. The interlayer is desired to have at least the following two characteristics: to have small light absorption coefficient in a wavelength range in which light in the bottom cell can be absorbed, and to have sufficient electric conductivity to prevent generation of large series resistance. And, materials that meet the requirements are desirably used.
Patent Document 1 discloses a solar cell comprising a plurality of cells of an p-i-n type or an n-i-p type structure of an amorphous Si or a crystalline Si laminated into a plurality of stages on a glass substrate via a transparent electrode layer, wherein at least a pair of adjacent cells partially contact with each other via an aperture hole in an insulation film formed between the cells, and Patent Document 1 exemplifies an oxide film, a nitride film, and a carbide film as the insulation film. With this configuration, according to the document, adjacent cells are point-contacted to enable reduction of shortening of the diffusion length of optically generated carriers due to a non-bonding hand at an interface between layers of each cell of different materials. In addition, such a configuration in which a plurality of cells are provided with an insulation film therebetween produces the above-mentioned effect of controlling the amount of incident light.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2003-124481